A Lesson Learned About Plastic Jars In The Microwave

By Joan Young on December 22nd
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This week, I defrosted some soup stock that I had stored in an empty plastic mayo jar. As you can see, the rim of the jar melted and burned! How did this happen?

My first mistake was probably to freeze the liquid in this jar. Although it seemed to be a great shape for the job, there is a small shoulder that prevents the frozen contents from sliding out. A better choice would have been a standard, slightly tapered freezer box. That’s why they are designed that way!

I did have some concerns about heating this jar, made of #1 plastic, polyethylene terephthalate (PET). Good Housekeeping has done extensive testing on the potential dangers of this plastic. Most things I’ve read say that the danger is almost nil if the plastic isn’t overheated, so I decided to just heat the jar on medium-low (defrost), just enough to be able to extract the broth. I know, if I had thought of it earlier in the day, I could have just defrosted it at room temperature.

After five minutes on defrost, I went to check the soup. I discovered the kitchen filled with the odor of burning plastic, and one black spot on the rim of the jar. I couldn’t see it until I removed the lid, which was not burned, at that point.

While I was looking at this, incredulously, I realized that there were scraps of the foil-backed paper still adhering to the rim of the jar, remnants of the seal that protected the contents from tampering. I had thought that these wouldn’t be a problem, especially since they were covered by the plastic jar lid, which I had loosened, but left on the jar.

I was quite surprised, because the US Department of Agriculture has said that under some conditions it’s even safe to use aluminum foil in a microwave. Some to-go soup and pasta containers you can buy, to heat in the office microwave, actually have a metal rim under their covers.

The contents still wouldn’t come out of my jar, so I took a knife and tried to scrape off the final shreds of that foil-backed paper. I thought I succeeded. But when I put the jar back in for another three minutes on defrost, the burned area enlarged, and another spot started. There was no sparking or arc-ing taking place inside the oven (which might have damaged the oven). However, those areas were clearly overheating and beginning to burn. As you can see, they burned right through the hard plastic lid. And this was all on the medium-low setting- never on high!

Needless to say, as soon as I could extract the contents, I stopped heating this container, now destined for the recycle bin.

The moral of the story? For me– to stop thinking that I’ve learned everything there is to know about kitchen safety. I would have told you, before this incident, that I never did unsafe things in my microwave. Now, I’ve learned one more safety tip. I thought I’d pass it along to you.